In recent years, SHM technologies have received increasing attention by industry as a potential method for improving the safety and reliability of structures and thereby reducing their operational cost. SHM is perceived as a revolutionary method for determining the integrity of structures involving the use of multidisciplinary fields, including sensors, materials, structure mechanics, signal processing and interpretation, and system integration. One thrust of this technology is the development of self-sufficient SHM systems for the continuous monitoring, inspection and damage detection of structures with minimal labor involvement. The aim of the technology is not simply to detect structural failures, but also to provide an early indication of physical damage that might give rise thereto. The early warning provided by a SHM system can then be used to define remedial strategies before the structural damage leads to failure. However, to ensure the reliability and robustness of SHM technologies for use in real-world applications, efficient design methodologies and implementation procedures are needed.
There are unique challenges in SHM which depend on the structural material and geometry, and the types of damages that are to be detected and monitored. In ultrasound Lamb wave based SHM, because of the dispersion characteristics, different wave packets in the received signals are sensitive to different types of damages, depending on the structure material. For example, in composite structures, impact damages could result in the largest change in the zero-order symmetric Lamb wave mode. This mode could be generally sufficient for impact damage detection in composite structures. This is different from metallic structures, where a crack damage could mostly alter the anti-symmetric Lamb wave mode while a corrosion damage could also alter the symmetric mode in the same time. In order to detect damage most effectively and to reduce false-acceptance rate, different wave modes should be used for different materials and problems. Therefore, a need exists to develop different damage detection methods for different applications.